Volume 1 Paper 4
An Electrochemical Investigation of the Corrosion Behavior of Aluminum Alloy AA5052 in Methanolic Solutions
L. A. Pawlick and R. G. Kelly
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JCSE Volume 1 Paper 4
Submitted 15 July 1995, revised version submitted 29 November 1995
An Electrochemical Investigation of the Corrosion Behavior of Aluminum
Alloy AA5052 in Methanolic Solutions
L. A. Pawlick, R. G. Kelly
Center for Electrochemical Science and Engineering
Department of Materials Science and Engineering
University of Virginia
Charlottesville, VA 22903
The electrochemical behavior of aluminum alloy AA5052 in methanolic
solutions containing low concentrations of acid, chloride, sulfate and water
has been studied. In all solutions investigated, the alloy exhibited
spontaneous passivity. The addition of 1 mM acid increased the corrosion
potential dramatically, whereas 1 mM chloride decreased the pitting potential
and 1mM sulfate had no measurable effects. Water appears to decrease the
pitting potential due to the enhancement of aluminum ion hydrolysis at
incipient pits. No important second order interactions were observed. The
effects of acid on the corrosion potential and rate are explained in terms of
mixed potential theory.
As the use of alcohol in automotive fuels increases due to environmental
concerns, so does the need to characterize and understand the corrosion of
metals and alloys in alcoholic solutions. Of particular importance are the
effects of low levels of the impurities commonly expected in these fuels such
as acid, chloride, sulfate, and water. In recent work on iron in methanolic
solutions, Brossia and Kelly first developed a quantitative characterization
of the electrochemical phenomenology of iron in methanolic solutions including
the effects of common impurities  which led to a description of the
mechanisms underlying the accelerating effects of acid and the inhibitory
effects of water on the open circuit corrosion rate . One of the key
elements of that work involved the use of statistical experimental design 
to allow a quantitative analysis of the main (first-order) effects of the
various solution additions, second-order interactions and an estimate of the
experimental error. The error estimation permitted the statistical
significance of each effect to be quantitatively assessed.
§3 Brossia et al.  were able to explain the two primary empirical
observations concerning the effect of acid on the corrosion of iron in
methanol [4-6]: large increases in corrosion rate (by a factor of 15 due to a
1 mM acid addition ) and the corrosion potential (by 200 mV due to a 1 mM
acid addition ). Brossia et al.  showed that the effects of acid were
due to two separable phenomena; acid activates the originally passive iron
surface, and proton reduction has substantially faster kinetics than oxygen
reduction in acidified methanol. This latter effect dominates, leading to an
increase in the corrosion potential with a subsequent increase in corrosion
§4 The addition of water inhibits the corrosion of iron in acidic methanol
dramatically [1-6]. Brossia et al.  determined that proton reduction was
under substantial mass transport control at the corrosion potential of iron.
They then showed that the addition of water to acidified solutions reduced the
corrosion rate predominantly by inhibiting the mobility of the proton, thus
reducing the proton diffusivity and hence the diffusion limited current
density of the dominant cathodic reaction in acidified methanol. This
reduction in diffusion limited current density led directly to the decrease in
the corrosion rate. The reduction in proton mobility is due to the
preferential solvation of protons by water relative to methanol . At low
concentrations of water, this limits the proton hopping that is important for
§5 In an extension of that work, the electrochemical behavior of aluminum
alloy AA5052 has been studied and analyzed. By studying a valve metal such as
aluminum which passivates with a thick film, information on the effect of the
nature of the passive film on the corrosion of materials in methanol can be
gained. In addition, AA5052 is being considered as a construction material for
future automobile fuel tanks. Thus, an assessment of the effects of impurities
on its corrosion behavior would have practical importance as well.
§6 Limited previous work on the corrosion of aluminum alloys in methanolic
solutions has been performed. Mansfeld  and Palit et al.  focussed on
the corrosion of aluminum alloys in strongly acidified methanolic solutions.
Upon anodic polarization of AA6061 above 0 V(SCE) in 0.1 N sulfuric acid,
Mansfeld observed pitting with gas evolution from the pits. He proposed that
the pitting was due to the presence of the sulfate ion. Palit et al.  also
observed pitting of pure Al in acidic methanol in solutions open to air.
Hronsky  measured weight loss for pure Al in 0.3 M HCl. Severe pitting was
observed under open circuit conditions. Wing et al.  and Lash  also
used coupon testing to investigate the behavior of various cast aluminum
alloys in M85 (15% fuel, 85% methanol with 1.1mM acid, 0.1% water, and 0.07mM
chloride). They found substantial weight losses and pitting of the cast Al
§7 In order to better understand the corrosion of aluminum alloys in methanol,
the present study was conducted. The corrosion and electrochemical behavior of
AA5052 in a variety of methanolic solutions was studied and the effects of the
addition of several impurities were quantified and explained in terms of the
previous work on iron . Previous observations in this area are also
rationalized within the framework of mixed potential theory.
Materials - Aluminum alloy AA5052 was supplied by Kobe Steel in the
form of 0.50 mm thick sheet. Its composition was (in wt.%) 2.46 Mn, 0.22 Fe,
0.19 Cr, 0.1 Si, 0.02 Ti, remainder Al. Disk electrodes were cut from the
sheet and wet polished with successively finer silicon carbide paper to a 600
§9 Solutions - All test solutions were based on spectrophotometry
grade, "Photrex" reagent methanol (J.T. Baker). All solutions
contained 0.1 M anhydrous sodium perchlorate (Aesar) as a supporting
electrolyte. Other solution additions included water, chloride, sulfate,
and/or acid. The chloride was added as anhydrous lithium chloride (Fisher),
the sulfate as anhydrous sodium sulfate (Aldrich), and acid as sulfuric acid.
The water content of the solutions was measured via Karl Fischer titration (Mettler
DL-18). A full factorial design was used to investigate the effects of the
various impurities. Two levels of concentration were used, zero and 1 mM,
except for water. The inherent water level of the methanol and that added from
perchlorate led to a minimum water content for solutions of <0.06 wt.%. The
effects of water were studied by the addition of water to a concentration of
wt.%. No efforts were made to remove dissolved molecular oxygen as this would
tend to evaporate large quantities of methanol, changing the concentrations of
the dissolved substances. Previous work has identified the relevant cathodic
reactions at occurring in the different solutions.
§10 Testing Procedures - All experiments were conducted at room
temperature. Electrochemical measurements were conducted with an E.G.&G.
Princeton Applied Research (PAR) Versastat(TM) controlled by the PAR Model 352
software. A silver/silver chloride (SSC) electrode immersed in a compartment
containing methanol with 0.1 M LiCl and 1.5 mass% water was used as the
reference electrode. The reference electrode was separated from the working
electrode solution by a Vycor(TM) frit. The SSC electrode has a potential of
-29 mV vs. an aqueous saturated calomel electrode. The SSC was found to be
very stable with respect to time and minimized the liquid junction potential
. Cyclic polarization scans were conducted at a scan rate of 0.5 mV/s
starting at an initial potential of -0.8 V(SSC). A vertex current density of 5
mA/cm2 was used. Automatic current interruption was used for on-line
correction for ohmic drop.
§11 Analysis Procedures - Each curve was analyzed for corrosion
potential, corrosion current density and pitting potential. The corrosion
current density was determined by use of PARcalc(TM) and checked manually by
Tafel extrapolation. To quantitatively determine the significance of the
effects of added impurities and the experimental error, the results of the full
factorial design were analyzed with Number Cruncher Statistical Software (NCSS)
ver.5.03 (licensed by J. L. Hintze). Performing duplicate experiments for each
condition and comparing the results of all experiments containing the solution
species of interest to all experiments that did not contain that species
allowed for the separation of effects due to single impurities as well as any
synergistic effects between species. Upon completion of the tests, each effect
was statistically analyzed to determine its significance as well as to
determine the experimental error.
Figure1: Cyclic polarization curve for AA5052 in the base solution (anhydrous
methanol containing 0.1 M sodium perchlorate).
Figure 2: Photomicrograph of AA5052 surface after polarization scan shown in Figure 1. Note the corrosion pits in an otherwise as-polished
In the base solution, AA5052 did not exhibit an active-passive transition
as shown in Figure 1. This lack of an
active-passive transition near the corrosion potential can be termed
spontaneous passivity, and this term is used throughout this manuscript to
refer to such behavior. This passive region was limited at +0.35 V(SSC) by
pitting as shown in the photomicrograph of Figure 2.
Figure 3: Polarization behavior for AA5052 in (a) the base solution and upon the
addition of individual impurities: (b) 1 mM acid or (c) 1 mM sulfate or (d) 1 mM chloride
or (e) 0.5wt.% water. Note the lack of an effect of sulfate, while acid, water and
chloride all lead to a decrease in the pitting potential.
The effects on the polarization behavior of the addition of individual
impurities (1 mM acid or 1 mM sulfate or 1 mM chloride or 0.5 wt.% water) are
shown in Figure 3. Acid increased the open circuit
potential and decreased the pitting potential of the AA5052, and increased the
open circuit corrosion rate slightly. The addition of water led to a decrease
in the pitting potential, while sulfate had little effect. Chloride decreased
the pitting potential.
§13 A summary of the first order effects of the impurities on the
electrochemical parameters that characterize the corrosion process in both low
and high water contents is shown in Figures 4 and 5.
Figure 4: Summary of the effects of the impurities (a) alone and (b) in combination
on the corrosion potential and open circuit corrosion rate for AA5052. All solutions
contained 0.1 M sodium perchlorate and <0.06 mass% water.
Figure 5: Summary of the effects of the impurities (a) alone and (b) in combination
on the corrosion potential and open circuit corrosion rate for AA5052. All solutions
contained 0.1 M sodium perchlorate and 0.5 mass% water.